Effects of Urbanization on the Dynamics of Organic Sediments in Temperate Lakes

Effects of Urbanization on theDynamics of Organic Sediments in

Temperate Lakes

Tessa B. Francis,1,* Daniel E. Schindler,2 Justin M. Fox,3 andElizabeth Seminet-Reneau4

1Department of Biology, and School of Aquatic and Fishery Sciences, University of Washington, P.O. Box 355020 Seattle, Washington

98195-5020, USA; 2School of Aquatic and Fishery Sciences, University of Washington, P.O. Box 355020 Seattle, Washington 98195-

5020, USA; 3Center for Limnology, University of Wisconsin, Madison, Wisconsin 53703, USA; 4Fishery Resources, University of Idaho,

Moscow, Idaho 83844-1136, USA

ABSTRACT

Residential development of lakeshores affects the

structure and function of riparian and littoral

habitats. Organic detritus in sediments is a critical

component of littoral food webs, but the effects of

urbanization on sediment characteristics are

unexplored. We characterized the quantity of or-

ganic sediments in Pacific Northwest lakes along a

development gradient and found a 10-fold decline

in the proportion of detritus in littoral sediments

associated with density of lakeshore dwellings. In a

comparison between two fully developed lakes and

two undeveloped reference lakes, we examined

several possible controls on sedimentary organic

content, including terrestrial inputs, decomposition

rates and associated macroinvertebrate communi-

ties, and physical retention by coarse wood. The

littoral sediments of undeveloped lakes ranged

from 34 to 77% organic by mass, whereas the

range on urban lakes was an order of magnitude

less, ranging from 1 to 3% organic. We found no

significant differences in terrestrial litter inputs

between the two sets of lakes. Leaf litter decom-

position rates did not vary significantly between

the two sets of lakes, and we found higher densities

of shredder macroinvertebrate taxa in the littoral

zones of undeveloped lakes. Sedimentary organic

matter on undeveloped lakes accumulated in

shallow waters and declined with distance from

shore, whereas the opposite pattern existed on

urban lakes. Our results suggest that coarse wood

physically retains organic matter in littoral zones

where it can enter the detrital energy pathway, and

the loss of this feature on urban lakes alters littoral

sediment characteristics, with potentially far-

reaching consequences for lake food webs.

Key words: littoral; urban ecology; coarse woody

debris; sediments; organic matter; leaf litter; aqua-

tic-terrestrial coupling; benthic invertebrates.

INTRODUCTION

As urban development sprawls, the effects of hu-

man activities on lake ecosystems are becoming

more apparent and complex. For several decades,

the greatest human threat to lake ecosystems was

eutrophication of surface waters (Schindler 1978;

Carpenter and others 1998). Though eutrophica-

tion owing to nonpoint source pollution from

agricultural, urban, and industrial sources remains

a problem for lakes in North America (Carpenter

and others 1998; Moore and others 2003), it is

apparent that human activities have myriad effects

Received 15 January 2007; accepted 26 June 2007; published online 31

July 2007.

*Corresponding author; e-mail: [email protected]

Ecosystems (2007) 10: 1057–1068DOI: 10.1007/s10021-007-9077-0

1057

on lake ecosystems. For example, fish growth rates

are negatively correlated with lakeshore develop-

ment intensity (Schindler and others 2000), and

the spatial distribution of fishes is altered by

urbanization (Lewin and others 2004; Scheuerell

and Schindler 2004). Lakeshore and watershed

urbanization are correlated with reduced amphib-

ian abundance (Woodford and Meyer 2003) and

shifts in benthic invertebrate (Quinlan and others

2002) and zooplankton (Dodson and others 2005)

community composition.

Human activities and their impacts on limnetic

ecosystems are concentrated along lakeshores,

where coupling between terrestrial and aquatic

systems is particularly strong. Residential develop-

ment of shorelines is associated with reductions in

riparian forest density as homes, lawns, and related

residential structures replace native vegetation

(Christensen and others 1996; Francis and Schin-

dler 2006; Marburg and others 2006) and alter

plant community diversity, reducing the abun-

dance of native plant species (Elias and Meyer

2003). Humans alter littoral habitats through resi-

dential development, for example by removing

macrophytes (Jennings and others 2003), which in

turn changes the spatial distribution of fishes

(Bryan and Scarnecchia 1992).

One unexplored consequence of lakeshore

urbanization is its impact on the distribution, com-

position, and abundance of sedimentary organic

matter. The role of detritus and organic matter as a

critical energy link in lakes has long been recog-

nized (Lindeman 1942). Recently, it is argued that

detritus forms the foundation for energy flow in

many aquatic ecosystems (Polis and Strong 1996;

Moore and others 2004). Organic matter provides a

substrate for colonization by bacteria and other

microbes, which are a food source for macroinver-

tebrates and fishes. Because benthic energy path-

ways are critical to upper trophic levels in lakes

(Schindler and Scheuerell 2002; Vander Zanden

and Vadeboncoeur 2002), the loss of this detrital

energy source could have major consequences for

food web structure and lake ecosystem function.

Here, we hypothesize that the loss of coarse

wood from urban littoral habitats increases the flow

of organic sediments to deeper waters, resulting in

lower organic content in littoral sediments. One

alternative hypothesis is that decreased inputs of

terrestrial leaf litter from thinned urban riparian

forests reduce littoral detritus, as development

intensity is also associated with riparian deforesta-

tion on this same set of lakes (Francis and Schindler

2006). Leaf litter inputs to lake surface waters have

seldom been quantified, and yet they can be sub-

stantial (Gasith and Hasler 1976; France and Peters

1995). Because allochthonous sources can domi-

nate the organic pools in littoral sediments (Piec-

zynska 1990a), it is possible that the losses of

particulate terrestrial inputs associated with ripar-

ian deforestation may be implicated in reduced

littoral detritus pools. Another potential explana-

tion for the lower organic content of urban lake

sediments is that developed lakes have higher rates

of decomposition, and therefore particulate organic

matter is more rapidly processed and transferred

into either the invertebrate pool or the dissolved

organic matter pool. Microbial degradation can

increase under higher nutrient conditions (Oertli

1993; Bayo and others 2005), and certain detritiv-

orous macroinvertebrates thrive under eutrophic

conditions associated with urbanization (Kashian

and Burton 2000). Higher rates of invertebrate- or

bacteria-driven decomposition in urban lakes may

therefore explain losses of littoral sediment organic

matter along a gradient of urbanization.

Loss of coarse wood (dead and downed tree

stems and branches, here defined as >1 m in length

and >10 cm in diameter) from littoral habitats is

strongly associated with shoreline urbanization

(Christensen and others 1996; Francis and Schin-

dler 2006; Marburg and others 2006). Although it

is assumed that wood is important in the structure

and function of littoral habitats (Schindler and

Scheuerell 2002; Jennings and others 2003), as yet

very little is known about the specific roles played

by coarse wood in lakes. It has been shown

extensively in streams that coarse wood increases

organic matter retention (Bilby 1981; Bilby and

Ward 1991), and less coarse wood in urban lakes

may result in a loss of sediment organic matter. The

majority of detritus in lakes is deposited in shallow

water zones (Piezynska 1990b) where it then be-

comes part of the sediments. In the absence of

coarse wood as a physical structure to retain par-

ticulate detritus in shallow waters, organic matter

deposited in littoral zones may be transported to

deeper waters, resulting in reduced organic content

of urban littoral sediments. This reduction in lit-

toral sediment quality may in turn have conse-

quences for upper trophic levels and lake-wide food

web interactions, as littoral habitats house a variety

of benthic invertebrates that serve as prey items for

fishes and are therefore key sites for benthic–pela-

gic coupling (Schindler and Scheuerell 2002; Van-

der Zanden and Vadeboncoeur 2002).

In this paper, we investigate the effects of shore-

line urbanization on littoral habitat characteristics,

specifically the organic content of littoral sediments.

We then explore several possible mechanisms to

1058 T. B. Francis and others

explain the relationship between lakeshore devel-

opment and sediment organic matter. Previous re-

sults showed a relationship between urbanization

and loss of littoral coarse wood (Francis and Schin-

dler 2006), therefore we mapped the distribution of

sediments with distance from shore, hypothesizing

that littoral coarse wood retains organic matter in

littoral sediments. Because the riparian zones of

urban lakes in this region are deforested (Francis

and Schindler 2006), we explored whether reduc-

tion in leaf litter inputs results in lower organic

matter in urban sediments compared to undevel-

oped lakes with intact riparian forests. Finally, be-

cause decomposition processes can control sediment

characteristics, we measured decomposition rates

on urban lakes and compared them to those ob-

served on undeveloped lakes to determine whether

biodegradation explains sediment organic content.

METHODS

We sampled 15 lakes in western Washington State

and southern British Columbia, Canada (see

Francis and Schindler 2006 for physical descrip-

tions). The lakes are all located in the western

hemlock (Tsuga heterophylla) zone of the Cascade

Range and Puget Trough regions. Study lakes span

a gradient of urban development and are mostly

located in the urban fringe of the Seattle Metro-

politan area. Nearly all lakes were closed to boats

with outboard motors. Three undeveloped lakes

were selected as reference systems, located in the

University of British Columbia‘s Malcolm Knapp

Research Forest. The latitudinal gradient between

our study sites is not sufficient to affect forest or

invertebrate community composition.

We surveyed the lakes between July and October

of 2002 and in October 2006 for development

intensity, coarse wood density and basal area, and

sediment composition. A full description of the

survey and coarse wood sampling strategy is given

in Francis and Schindler (2006). On each lake, we

selected 4–8 transects measuring 30 m along the

lakeshore, ensuring to select an even number of

both leeward and windward sites. Along each

transect, we enumerated and measured the diam-

eter of all pieces of coarse wood (‡10 cm in diam-

eter, ‡1 m in length) within or intersecting the

0.5 m depth contour. We calculated coarse wood

basal area for each plot as the sum of all individual

log basal areas, defined as

basal area¼ p�r2 ð1Þ

where r = radius at breast height (trees), or where

the log intersected the 0.5 m depth contour. We

characterized sediment composition by collecting

the top 5 cm of sediments in 0.5 m of water using a

10 cm diameter hand-held sediment corer. Sedi-

ments were refrigerated to limit further biotic

processing until they were dried at 60�C to a con-

stant mass and then combusted at 550�C to deter-

mine ash free dry mass (AFDM). Sediment organic

matter was calculated as the proportion of dry mass

lost during combustion.

To investigate the mechanisms driving the rela-

tionship between lakeshore urbanization and sed-

iment organic matter observed in the survey, we

conducted a focused comparative study between

two urban, fully developed lakes (that is, >95% of

shoreline developed; Star and Shady Lakes) located

in suburban areas of Seattle and two wholly

undeveloped lakes (Loon and Gwendoline Lakes)

in British Columbia (Figure 1). These two sets of

lakes represent the high and low ends of the gra-

dients for sediment organic matter and residential

development observed in the survey (Francis and

Schindler 2006; Table 1).

We measured leaf litter input on the set of four

lakes by establishing three transects running per-

pendicular to shore at randomly selected sites along

the shoreline. Along each transect, we measured

the lateral distribution of terrestrial leaf litter inputs

to surface waters from July 2003 to April 2004 by

deploying litter traps, 20 cm · 30 cm (0.06 m2)

floating plastic bins, at 1, 5, 10, 20, and 40 m from

shore. The litter traps passively sampled aerial lit-

terfall constantly for 10 months. Contents were

removed every 22–61 (median: 46) days and dried

at 60�C to a constant mass. In September 2003, a

heavy rainstorm raised the water level of Loon

Lake such that all traps were swamped. In February

2004, Gwendoline Lake was frozen and traps were

inaccessible; however, bins continued to collect

litter inputs, and therefore the sample collected in

March was assumed to include inputs from the

previous period.

To measure decomposition rates in the four

lakes, we incubated red alder (Alnus rubra) leaves

in mesh bags anchored to the lake bottom at two

depths during late summer/autumn and winter.

We collected senescent alder leaves from the wa-

tershed surrounding each lake, dried them at room

temperature, and created leaf packets which were

placed in either fine (<0.5 mm) or coarse (1 cm)

mesh bags. The fine mesh bags successfully pre-

vented the colonization of leaf packets by macro-

invertebrates. Bags were weighed before

incubation and anchored to cinder blocks on the

lake bottom at two depths, above and below the

thermocline, to incorporate potential variation in

Urbanization and Lake Sediments 1059

invertebrate and microbial decomposition rates

associated with temperature. Several reference bags

were put through the weighing, bagging, and

placement process to establish mass lost during

transport, and this correction factor ()1.6% for fine

bags, )6.9% for coarse mesh) was applied to all

bags before statistical analyses. After the incubation

period, bags were retrieved by SCUBA, dried at

60�C to a constant weight, and reweighed.

Decomposition rate k (Petersen and Cummins

1974) was calculated according to an exponential

function as follows:

Wt ¼ W0eð�ktÞ ð2Þ

where Wt is equal to the mass of leaf material

remaining after incubating for t days and W0 is the

initial corrected mass.

We collected surface sediments at various dis-

tances from shore on all four lakes to assess the

spatial distribution and particle size composition of

sediment organic matter. We stratified distances

into ‘‘near‘‘ (1–10 m) and ‘‘far‘‘ (20–40 m) from

shore, based on previous work showing that the

majority of allochthonous inputs are deposited

within 10 m from shore (France and Peters 1995).

We did not measure sediment organic matter in the

deep basins of these lakes, but rather concentrated

on the littoral zones, where organic matter accu-

mulation was observed in the survey. We collected

sediments using an Ekman dredge below each

floating litter trap in summer and winter. Each

sample was sorted using soil sieves into smaller

than 0.06, 0.06, 0.12, 0.18, 0.42, 1.0, 2.5, and

6.35 mm size categories and dried at 60�C to a

constant mass. A subsample of each was combusted

and the proportion of organic content was calcu-

lated for each size category as described above.

We sampled the benthic macroinvertebrate

communities in each of the four lakes along the

established transects using an Ekman dredge dur-

ing the late summer of 2003. We collected inver-

tebrates in surface sediments below each floating

litter trap at depths of 0.3–11 m of water. Inverte-

brates were preserved in 95% ethanol and identi-

fied to family.

Statistical Analyses

We used SYSTAT 11.0 (Systat Software Inc., 2004)

for all statistical analyses. Data transforma-

tions—arcsine square root transformations for the

proportion organic matter in sediments and natural

log transformation for density of lakeshore resi-

dences—were performed prior to statistical analy-

ses to normalize data. We used least-squares

regression using data from the 15-lake survey

(excluding one outlier; Studentized resid-

ual = )3.1) to assess the relationships between

lakeshore residential density and sediment organic

matter, and coarse wood density and sediment or-

ganic matter content. Previous work has demon-

strated a relationship between lake morphometry

and sediment organic matter (Rowan and others

Figure 1. Shorelines of Pacific

Northwest lakes of different residential

development intensity. A Gwendoline

Lake, British Columbia, undeveloped;

B Shady Lake, Washington State, 100%

of shoreline developed.

Table 1. Lake Biophysical Characteristics

Lake Location Maximum

depth (m)

Mean

depth

(m)

Surface

area

(ha)

Residential

density

(houses m)1

shoreline)

Coarse wood

density

(pieces m)1

shoreline)

Sediment

organic matter

(proportion

by mass)

Epilimnetic

Chlorophyll-a

(ll)1)

Gwendoline 19�49¢N 34�122¢W 27 13.4 13.0 0 425.0 0.77 0.69

Loon 19�49¢N 34�122¢W 55 26 48.6 0 516.7 0.34 0.66

Shady 25�47¢N 6�122¢W 12 6 8.5 40 10.0 0.01 4.83

Star 21�47¢N 17�122¢W 15 8 14.2 40.6 0 0.03 1.32


1992), therefore we also tested for a relationship

between lake size (surface area and perimeter) and

sediment organic matter in the 15 lakes using least-

squares regression. A final regression investigated

the relationship between lake size (surface area)

and residential density. In the focused comparison

between urban and undeveloped lakes, we tested

whether decomposition rates varied according to

lake development level using ANOVA with daily

mass loss rate as the response variable and devel-

opment level (developed and undeveloped), season

(summer and winter), depth (above and below the

thermocline), and mesh size (coarse and fine) as

main factors, using post-hoc tests where we found

significance. To test the effect of coarse wood on

organic sediment particle distribution, we used

ANOVA with coarse wood density (high, low) and

distance from shore as main effects on organic

content of each particle size category, and we ran

Student‘s t-tests to test for development differences

within seasons at specific distances. We used a

MANOVA to test for significant differences in

invertebrate communities between the undevel-

oped and urban lakes.

RESULTS

Sediment organic matter decreased significantly as

shoreline residential development density in-

creased in the survey of 15 lakes (adjusted

r2 = 0.60, n = 15, P = 0.001; Figure 2A). The sedi-

ment organic content varied from roughly 74% on

undeveloped lakes to less than 2% on fully devel-

oped lakes. The organic matter content of littoral

sediments was significantly and positively associ-

ated with the density of coarse wood (adjusted

r2 = 0.63, n = 14, P = 0.001; Figure 2B) and coarse

wood basal area (adjusted r2 = 0.68, n = 14,

P < 0.001; not shown). We found no significant

relationship between sediment organic matter and

either metric of lake size, surface area (adjusted

r2 = 0.07, n = 14, P = 0.18; Figure 2C), or shore-

line length (adjusted r2 < 0.001, n = 14, P = 0.99),

and we found no significant relationship between

residential density and lake surface area (adjusted

r2 < 0.001, n = 14, P = 0.78; Figure 2D) or lake

perimeter (adjusted r2 = 0.07, n = 14, P = 0.15; not

shown).

On each of the four lakes sampled for the fo-

cused comparison, annual terrestrial leaf litter

inputs decreased with distance from shore (Fig-

ure 3A). We found no significant effect of

development (F1,17 = 2.5, P = 0.13) on annual

leaf litter inputs to surface waters across all dis-

tances combined, though the trend was for

higher inputs to urban lakes than to undeveloped

lakes (Figure 3A; Table 2). Litter inputs varied

with distance on all lakes (F1,17 = 5.1, P = 0.04),

decreasing with distance from shore according to

a negative exponential model (developed

lakes: Inputs = 158.98e)0.31 · Distance, r2 = 0.98;

undeveloped lakes: Inputs = 66.9e)0.67 · Distance,

r2 = 0.99). The majority (that is, >70%) of leaf

litter was deposited within 5 m of shore, and

Figure 2. Organic proportion of littoral

sediments as a function of A residential

development and B coarse wood

density; and sediment organic

proportion C, and density of shoreline

residences D as a function of lake

surface area. All data shown are from a

survey of 15 Pacific Northwest lakes,

excluding one outlier (n = 14).

Proportion data were arcsine-square

root transformed prior to statistical

analysis. Each point represents a whole

lake mean.


significantly less was observed beyond 10 m

(P = 0.04). On a seasonal basis, leaf litter inputs

varied significantly by development only during

fall (P = 0.04; Figure 3B and Table 2), with

higher inputs to developed lakes during that

season. Peak seasonal leaf litter inputs occurred

during fall on developed lakes and during sum-

mer on undeveloped lakes.

We found significant effects of season and

mesh size, but no significant effect of lake

development or depth relative to the thermocline,

on decomposition rates across all litterbag treat-

ments (Table 3; Figure 4). Decomposition rates

were greater in summer than in winter (Tukey‘s

HSD adjusted r2 = 0.22, df = 1, P = 0.006), and

were greater in coarse mesh bags than in fine

(Tukey‘s HSD adjusted r2 = 0.53, df = 1,

P < 0.001), across both development types and

incubation positions.

In undeveloped lakes, sediment organic matter

accumulated inshore, whereas in developed lakes,

sediment detritus increased with distance away

from shore (Figure 5). We found significant

interactions between development and distance

from shore (F4,19 = 22.4, P < 0.0001) and distance

and season (F4,19 = 4.1, P = 0.02), and we found

a significant three-way interaction effect between

development, season, and distance from shore

(F4,19 = 5.3, P = 0.005) on the organic proportion

of sediments between 1 and 40 m from the shore,

such that organic matter decreased with distance

from shore on undeveloped lakes but increased

with distance on developed lakes. These patterns

generally held in both summer and winter, ex-

cept that overall organic content was higher in

urban lakes in winter and more variable closest to

shore in undeveloped lakes in summer. We also

found significant effects of development

(F1,19 = 86.0, P < 0.0001) and season (F1,19 = 8.7,

P = 0.008) on the proportion organic in sedi-

ments. The mean (SE) proportion by mass of

organic littoral sediments on undeveloped lakes

was 0.71 (0.11), as compared to 0.11 (0.05) on

developed lakes. In both seasons, the amount of

sediment detritus on the two lake types con-

verged at 40 m from shore.

Urbanization affected the size distribution of

sediment particles. In particular, the smallest par-

ticles (<0.06 mm) accumulated nearshore on

undeveloped lakes in greater proportions than on

developed lakes (Figure 6). In fact, there were al-

most no fine particles in the nearshore sediments of

developed lakes. In tests on the distribution of the

smallest (<0.06 mm) organic sediment particles,

there were significant interaction effects between

development and season (F4,20 = 7.3, P = 0.01) and

distance and season (F4,20 = 4.2, P = 0.01), as well

as a significant distance effect (F4,20 = 23.8,

P < 0.0001). In summer, a significantly greater

proportion of fine organic particles accumulated

within 5 m of shore on undeveloped lakes versus

developed lakes (P = 0.02). In contrast, there was

no significant difference in the proportion of fine

organic particles in littoral sediments between

developed and undeveloped lakes in winter

(P = 0.85).

The benthic macroinvertebrate community var-

ied substantially between urban and undeveloped

lakes (Figure 7). The macroinvertebrate commu-

nity composition in developed and undeveloped

lakes were significantly different (MANOVA, Wilks‘

Lambda, F7,45 = 2.18, P = 0.05); specifically, we

found higher densities of shredding caddisflies

(Trichoptera) on the undeveloped lakes (P = 0.02)

and higher densities of detritivorous isopods (Iso-

poda) on urban lakes (P = 0.03; Figure 7). The

densities of grazing mayflies (Ephemeroptera) and

predatory odonates (Odonata) were 12.9 and

10.7 m)2, respectively, in the littoral zones of

Figure 3. Terrestrial leaf litter inputs to nearshore sur-

face waters on two undeveloped (solid bars) and two

developed (open bars) lakes. A Annual inputs (±1 SE) at

varying distances from shore. B Seasonal inputs (±1 SE)

across all distances.


undeveloped lakes, whereas these taxa were en-

tirely absent from similar habitats on urban lakes.

DISCUSSION

Shoreline urbanization has a variety of impacts on

lake ecosystems, including eutrophication and al-

tered littoral habitat structure, vegetation commu-

nity composition, and fish growth and behavior

(Schindler and others 2000; Scheuerell and

Schindler 2004; Marburg and others 2006). Our

results demonstrate two additional consequences of

shoreline urbanization: changes in the composition

and distribution of organic sediments and shifts in

the benthic macroinvertebrate community. We

suggest that the best explanation for depleted or-

ganic matter in urban littoral habitats is the absence

of coarse wood, which provides a physical structure

that retains detritus in littoral zones of undevel-

oped lakes. Without coarse wood to retain organic

matter in shallow waters, organic matter is trans-

ported by gravity and hydrodynamics to deeper

waters on urban lakes.

Riparian Litterfall Influence on SedimentOrganic Matter

Our finding that leaf inputs on all lakes decrease

with distance from shore according to a negative

exponential model concurs with previous studies

(Szczepanski 1965; Rau 1976); however, we did not

find reduced annual litter inputs to developed lakes.

Based on other findings that terrestrial inputs can be

tightly linked to littoral organic matter (Efford 1969)

and the extensive literature on the role of litterfall in

lotic ecosystems, we expected that litterfall into

lakes would be reduced in urban environments,

where dramatic riparian deforestation has occurred

(Francis and Schindler 2006). Indeed, the rare

quantification of litterfall to the surface waters of

lakes has been expressed in terms of litter inputs per

Table 2. Leaf Litter Inputs to Surface Waters of Two Developed (Star, Shady) and Two Undeveloped(Gwendoline, Loon) Lakes

Annual inputs

(g dry weight)

Seasonal inputs (g dry weight)

Mean SE Summer Late summer/early fall Fall Winter/early spring

Developed 165.1 63.7 8.9 21.4 99.7a 10.8

Undeveloped 37.4 21.2 19.6 4.3 13.1b 0.0

Different letters in each column indicate significant differences among lake types (Student‘s t-test; P < 0.05).

Table 3. Summary of ANOVA Results showingEffects of Development (developed, undeveloped),Depth (above, below the thermocline), Season(summer, winter) and Bag Mesh (coarse, fine) onLeaf Litter Decomposition Rates

Factor Least squares mean F-ratio df P-value

Development 0.2 1 0.70

Undeveloped 0.004

Developed 0.004

Depth 3.3 1 0.09

Above 0.0044

Below 0.0035

Season 23.5 1 0.0002

Summer 0.005

Winter 0.003

Mesh 55.9 1 <0.0001

Fine 0.002

Coarse 0.006

Figure 4. Daily decomposition rates of leaves incubated

in two undeveloped (filled boxes) and two developed (open

boxes) lakes in coarse and fine mesh bags during summer

and winter. Boxes represent whole lake means.


meter of forested shoreline (Szczepanski 1965;

Jordan and Likens 1975; Gasith and Hasler 1976).

One explanation for the absence of significantly

different litterfall rates between developed and

undeveloped lakes is the riparian forest composition

shift that occurs with urbanization on lakes in this

region. In lowland areas of the Pacific Northwest, the

native riparian forest is dominated by conifers, pri-

marily Douglas-fir (Pseudotsuga menziesii), western

hemlock (Tsuga heterophylla), and western red cedar

(Thuja plicata; Franklin and Dyrness 1973). On the

urban lakes in this study, in contrast, native forests

are most often replaced with non-native, deciduous

species (T. Francis, personal observation), which

have considerably higher litterfall rates than conifers

(Binkley and others 1992). In regions where native

forests are dominated by deciduous tree species,

deforestation associated with urbanization may re-

duce organic inputs to littoral zones of urban lakes,

resulting in even greater declines in littoral detritus.

Decomposition of Sediment OrganicMatter

We observed no consistent differences in decom-

position rates between urban and undeveloped

lakes that might explain either higher total OM or

greater proportion of fine particulate OM (FPOM)

in undeveloped lakes. Biological reduction of the

coarse particulate OM (CPOM) pool occurs through

processing and ingestion by macroinvertebrates

and via microbial degradation. Both processes re-

duce CPOM while increasing the FPOM pool

(Saunders and others 1980). Despite higher densi-

ties in the undeveloped lakes of shredding caddis-

flies, which are more effective decomposers relative

to other shredder taxa (Bjelke and Herrmann

2005), decomposition in the presences of macro-

invertebrates was not significantly higher on

undeveloped lakes compared to urban lakes in any

of the treatment regimes. In addition, the trend

towards higher densities on undeveloped lakes of

detritivorous amphipods, which ingest FPOM,

could only potentially decrease the fine organic

pool in these systems.

We found no difference in decomposition be-

tween urban and undeveloped lakes despite vari-

ation in some abiotic characteristics of the two sets

of lakes. Beyond biotic activities, decomposition

rates vary according to temperature, oxygen con-

centration, nutrient levels, substrate, particle size,

turbulence, and pH. Our study lakes are similar in

terms of annual temperature regimes, though the

undeveloped lakes have a slightly higher tendency

to freeze for short periods of time in winter. The

urban lakes are mesotrophic (total epilimnetic

P = 9–10 lg l)1), whereas the undeveloped lakes

are oligotrophic (epilimnetic P = 3–4 lg l)1; Moore

and others 2003). Despite elevated nutrient levels,

however, we observed no greater overall decom-

position rates in the urban lakes.

Decomposition of particulate organic matter

varies across substrate type. We did not directly

measure the source contributions of littoral detritus

in each lake, and the trophic status of the urban

lakes suggests there might be a greater proportion

of autochthonous POM, which is more labile and

could be processed more rapidly (Hicks and others

1994), reducing the total OM pool. Given the

equivalent leaf litter inputs in both lake types,

however, different detritus composition does not

explain the pattern of organic matter we observed.

Decomposition rates of terrestrial leaf litter also

vary by litter type. For example, deciduous alder

leaves decompose more quickly than more re-

calcitrant conifer needles (Webster and Benfield

Figure 5. Proportion organic matter in surface sediments

with distance from shore on two undeveloped (filled

symbols) and two developed (open symbols) lakes in sum-

mer (A) and winter (B). Error bars are 1 SE. Asterisks

indicate statistically significant differences (Student‘s t-

test, P < 0.05).


1986). The riparian forest community on the

undeveloped lakes is dominated by conifers, versus

deciduous trees on urban lakes. Thus, it may be

that urban litter is decomposed more quickly,

leaving less total organic matter in shallow water

sediments. Increased litter processing, however,

would result in an accumulation of FPOM in

nearshore sediments on urban lakes, which we did

not observe.

We observed striking differences in the macro-

invertebrate community composition between ur-

ban and undeveloped lakes that may be associated

with reductions in coarse wood and organic sedi-

ments. In addition to the differences stated above in

the shredder taxa, we found a complete loss of

predatory odonates (Odonata) and grazing mayflies

(Ephemeroptera) from the urban lakes. The ab-

sence of these taxa may be associated with a suite

of alterations to urban littoral habitats, including

the loss of coarse wood, which serves as habitat,

food resource, and predation refuge. These larger

invertebrate taxa are key prey for native trout, and

their absence may portend a decline in the ability

of urban lakes to support productive native fish

populations (Schindler and others 2000).

Organic Matter Distribution andRetention

Overall, the amount of organic material in littoral

sediments in undeveloped lakes was significantly

greater than in urban lakes. Sediment organic

matter accumulated in shallow waters on undev-

eloped lakes and decreased with distance from the

shore, but on urban lakes, organic matter increased

with distance from the shore. In the 15-lake sur-

vey, we found a significant positive correlation

between the proportion of organic matter in littoral

sediments and coarse wood density, and in the fo-

cused comparison, coarse wood density was sig-

nificantly greater on the two undeveloped lakes

compared to the urban lakes. Based on these data

and the organic particle distribution, we suggest

that littoral OM accumulation results from physical

retention by coarse wood that impedes sediment

focusing and retains POM in shallow waters. This

build-up of organic matter around wood has been

extensively observed in lotic ecosystems, where

Figure 6. Size composition of organic

sediment particles with distance away

from shore on four Pacific Northwest

lakes in summer. Particles are separated

into size categories: smaller than 0.06,

0.06, 0.12, 0.18, 0.42, 1.0, 2.5 and

6.35 mm. Distributions shown are

organic proportions by mass of surface

sediments.

Figure 7. Macroinvertebrate densities on two undevel-

oped (solid bars) and two developed (open bars) lakes.

Asterisks indicate statistically significant differences (Stu-

dent‘s t-test, P < 0.05), error bars are standard errors.


large wood adds hydraulic complexity to rivers and

streams (Bilby and Ward 1991; Bisson and Bilby

1998) and retains both coarse and fine organic

matter (Bilby 1981; Harmon and others 1986)

where it is subjected to biodegradation processes.

The generality of the relationship we have found

here between woody debris and sediment organic

matter relies on, among other things, inputs of

woody debris and production of organic matter.

Accumulation of sediment organic matter in littoral

zones is regularly observed in shoreline areas with

submerged vegetation (James and Barko 1990;

Wetzel 1990). Littoral-wetland zones are areas of

high organic matter loading and degradation

(Pieczynska 1990a; Wetzel 1990; Gude and others

2004); hence, accumulations at lake margins in

these systems are common despite the absence of

coarse wood. Lakes with significant portions of

riparian habitat in wetlands, desert, agriculture,

tundra, or otherwise lacking forest would not re-

ceive the inputs of woody debris found in these

lakes and would not fit the pattern we have ob-

served. Likewise, lakes with unforested watersheds,

such as alpine systems, may receive lower inputs of

particulate OM and not accumulate littoral detritus

to the same degree as the lakes sampled here. Al-

though the evidence is limited, existing research

shows that sediment OM is not a function of lake

trophic status (Rowan and others 1992). Instead,

sediment OM in lakes has been correlated with

maximum lake depth, lake surface area and catch-

ment area: lake area (Rowan and others 1992), al-

though we did not find such relationships here.

Human Behavior and Littoral Sediments

We did not directly monitor human behavior, so

we cannot exclude the potential that the alteration

of shorelines and littoral zones by urban lake resi-

dents contributes to the sediment patterns we ob-

served. Certainly human behavior affects the

density of coarse wood on urban lakes, as people

clear littoral zones for swimming and other recre-

ational activities. Furthermore, residents often de-

posit gravel or other non-organic substrates along

their shoreline, creating a homogenous, non-or-

ganic littoral zone (T. Francis, personal observa-

tion). Our sampling, however, captured a variety of

residential littoral zone types, including those with

gravel and those without. Another human influ-

ence on sediment composition is erosion from

lakeshore development which likely mobilizes soils

into lakes. When these lakes were initially defor-

ested at the turn of the twentieth century, as well

as during subsequent development for residential

use, there may have been intrusions of terrestrial

carbon into the lakes. These patterns could be

elucidated through paleolimnological investigation.

In addition, sediment distribution is associated with

wave action and boating can induce wave-like

water movements, even on small lakes. The urban

lakes in this study, however, restricted the use of

motors, allowing only those that propel boats at

speeds that do not generate waves. Nevertheless,

although we have observed a strong correlation

between sediment organic matter and coarse wood

density, we cannot definitively rule out human-

induced changes to littoral sediments in contrib-

uting to these patterns.

IMPLICATIONS

Most lakes in the world are net heterotrophic (Cole

and others 1994, 2000; Hanson and others 2003).

The respiration of carbon in lakes is concentrated in

littoral habitats, which are conducive to decompo-

sition and other biodegradation processes because

they are warm and oxygenated (Pieczynska 1990b).

If the presence of littoral woody debris enhances

degradation processes by retaining organic carbon

in the littoral zone, coarse wood may contribute,

albeit to an unknown extent, to lake heterotrophy

and shift the balance of carbon storage in lake sed-

iments relative to evasion to the atmosphere.

Loss of organic matter from littoral sediments has

important implications for food web structure and

ecosystem function in urban lakes. Allochthonous

particulate carbon inputs are generally considered

to support only a minor fraction of lake produc-

tivity (for example, Saunders and others 1980; and

references), yet there is some evidence that it can

be vital to secondary production in some lakes

(Pace and others 2004; Cole and others 2006).

Terrestrially derived detritus accumulates in the

shallow waters of lakes in greater amounts than

living material, and in this way acts as an energy

reservoir (Saunders and others 1980), fuelling

microbial processes and secondary production of

aquatic invertebrates that in turn support upper

trophic levels. Because most lentic biota are asso-

ciated to some degree with littoral habitats, it is

likely that the loss of coarse wood and the resulting

reduction in organic matter destabilizes lake food

webs via reduced littoral secondary productivity

and subsequent trophic decoupling. In particular,

our results suggest a decline in the density of

benthic macroinvertebrates in urban lakes, which

is likely to impair the ability of urban lakes to

support fish that rely heavily on benthic resources.


ACKNOWLEDGMENTS

This work was supported by the National Science

Foundation (IGERT-0114351) and the University

of Washington‘s College of Forest Resources. We

thank the staff of the UBC Malcolm Knapp Re-

search Forest for their logistical assistance and ac-

cess to Loon and Gwendoline Lakes. We are also

grateful to the residents of Star and Shady Lakes for

their assistance and patience. Matt Baker, P. Dee

Boersma, Gordon Holtgrieve, John Marzluff, and

Mark Scheuerell provided helpful comments on

earlier drafts of the article.

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